Beyond that, i don't think there are any particularly good recent guides. Unfortunately, TG has a really backwards policy towards external sites, so external help sites don't really happen anymore, or don't people don't bother updating them cause it isn't worth the effort.

you could start with ->this guide. Its very beginner oriented (what you might or might not be looking for), but it gives you an idea whats the deal with gauls and how a good start with them could look like.
I also kinda like this and that from the wiki.

There is not a one fits all solution how to play gauls (or the two other tribes), as its highly depending on what exactly you want to play and achieve and how the server unfolds